It is known that commercial vehicles are provided with a gear change with a high number of gear ratios, for example eight, twelve or sixteen forward gear ratios and one or more reverse gear ratios.
The known gear changes generally comprise two or three groups or stages arranged in cascade with one another and each defining a reduced number (from two to four) of transmission ratios; therefore, the total number of gear ratios of the gear change is defined by the product of the transmission ratios of the individual stages.
A typical configuration is one comprising a main group or "main box" with four ratios plus reverse and at least one auxiliary group or "range" disposed downstream of the main stage and defining two transmission ratios for a total of eight gear ratios. The transmission ratios of the auxiliary group are calculated so as to define, in combination with the transmission ratios of the main group, two sets of gear ratios (I,II,III,IV; V,VII,VIII) which are separate and adjacent one another.
The selection and engagement of the gear ratios are controlled by a manually operated lever, the control grid of which (i.e. the entirety of the paths along which the lever has to be moved to select and disengage the various gear ratios) becomes increasingly complex as the number of gear ratios increases.
Referring to the example illustrated above (eight forward gear ratios), the grid may be of the so-called "double H" type; the gear ratios are disposed in pairs (I and II; III and IV; etc.) in respective engagement planes which can be selected by moving the lever along a selection plane (generally transversal with respect to the direction of travel of the vehicle); the gear ratios associated with the selected engagement plane can be engaged by subsequently moving the lever along the engagement plane forwards (odd gear ratios) or rearwards (even gear ratios) with respect to the selection plane.
Since the overall dimensions of the grid are limited for ergonomic and space reasons, as the number of gear ratios increases inevitable the distance between the various engagement planes decreases; in operation, this may give rise to the possibility of selection errors in the engagement plane, resulting in the engagement of incorrect gear ratios.
With the aim of resolving this problem selector levers with a "repeated H" grid have been proposed; in this case the grid is a simple H but the lever is provided with a selector which can move vertically making it possible to "split" the grid itself into two superimposed planes, to each of which corresponds a respective transmission ratio of the auxiliary group. The selection of the transmission ratios of the main group takes place in each of the two planes, said selection being easy and reliable since the distance between the planes of engagement is greater than in the case of a corresponding double H grid.
However, this arrangement also has drawbacks since it is possible for the driver to move the selector inadvertently into the upper level or lower lever, or vice versa, when changing gear, which in some cases may have serious consequences both for driving safety and for the mechanical integrity of the power plant. For example, when changing gear so as to "drop down" from the eighth gear ratio an error of this type could result in the engagement of third gear instead of seventh gear, of second gear instead of sixth gear, with the obvious risks of causing the engine to run at a speed of rotation greatly exceeding the limits of mechanical strength.
To resolve this problem electronic control systems have been proposed, which at the input receive a plurality of signals relating to the operating parameters of the gear change (speed of rotation of the input and output shafts, gear ratio engaged, gear ratio selected, etc.), and which process said input signals so as to enable or prevent the operation of the selector on the basis of stored maps. However, such systems entail both the use of a complicated and costly control unit and the installation of additional components for the gear change (mechanical locking elements, sensors and associated wiring).
Moreover, systems of the above-mentioned type are not always reliable and may not intervene at the right time in the case of particularly rapid maneuvers.